The NADH Reoxidation Issue

ANODIC CATALYSTS FOR OXIDATION OF CARBON-CONTAINING FUELS 

The electrochemistry of this cofactor in both oxidized and reduced forms is irreversible and conveys an additional complexity in fuel cell design. Electrochemical reduction of NAD requires specific conditions in order to limit or avoid adsorption of NAD that causes enzyme inhibition, reportedly due to the adenine moiety of the cofactor [27-32]. 

The mechanism for electrochemical oxidation of NADH has been proposed in a number of studies as an electrochemical-chemical-electrochemical mechanism characterized by the following reaction scheme (Equation 4.1) [32-34]  

The high overpotential of the direct electrochemical oxidation of NADH described in Equation 4.1 is caused by the very high potential of the N AD /NADH redox couple (the first step in the reaction) [35], which results in an initial rate-limiting electron transfer step. It has also been suggested that the intermediate radicals resulting from the first and second steps of this reaction might participate in alternative reaction routes for electrochemical oxidation of NADH [34]. 

To summarize, the direct electrochemical reactions of the NAD /NADH cofactors at metallic or carbon electrodes are highly irreversible, occur at large overpotentials, and can be affected by side reactions and fouling (adsorption) of cofactor-related products [26]. The development of BFCs based on NAD -dependent dehydrogenases, therefore, cannot rely on direct electrochemical reactions of the cofactor. As a result, a major focus for this field of research has been directed at finding suitable methods for the electrochemical oxidation of NADH. 

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